The term "sexual instinct" may be said to cover the whole of the
neuropsychic phenomena of reproduction which man shares with the lower
animals. It is true that much discussion has taken place concerning the
proper use of the term "instinct," and some definitions of instinctive
action would appear to exclude the essential mechanism of the process
whereby sexual reproduction is assured. Such definitions scarcely seem
legitimate, and are certainly unfortunate. Herbert Spencer's definition of
instinct as "compound reflex action" is sufficiently clear and definite
for ordinary use.
It may be said that the whole of the task undertaken in these Studies is
really an attempt to analyze what is commonly called the sexual instinct. In order to grasp it we have to break it up into its component parts. Lloyd Morgan has pointed out that the components of an instinct may be regarded as four: first, the internal messages giving rise to the impulse; secondly, the external stimuli which co-operate with the impulse to affect the nervous centers; thirdly, the active response due to the co-ordinate outgoing discharges; and, fourthly, the message from the organs concerned in the behavior by which the central nervous system is further affected.
In dealing with the sexual instinct the first two factors are those which
we have most fully to discuss. With the external stimuli we shall be
concerned in a future volume (IV). We may here confine ourselves mainly to
the first factor: the nature of the internal messages which prompt the
sexual act. We may, in other words, attempt to analyze the sexual impulse.